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3.0 HEREDITY AND VARIATION
3.1 CELL DIVISION
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3.1.1 Genes, Deoxyribonucleic acid 3.1.1 Genes, Deoxyribonucleic acid
(DNA) and Chromosomes(DNA) and Chromosomes
• Chromosomes are a long, fine, thread-like structures found in the nucleus
• It builds from molecules called deoxyribonucleic acid (DNA)
• Every species has its own number of chromosomes per cell to store their heredity information
• Example humans possess 46 chromosomes
• 2 types of chromosomes:
1. Autosomes – controls all somatic
traits (blood group, body weight &
skin colour)
2. Sex chromosomes – determine
gender
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• In human:
44 autosomes + 2 sex chromosomes = 46
chromosomes
• 2 types of sex chromosomes:
XY – chromosomes in males
XX – chromosomes in females
Male Chromosomes
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• Genes are basic heredity unit
• Genes control the development of traits (e.g. genes that control body height and skin texture)
• Each gene controls one trait that are inherit from parents
• Inherited characteristics are passed from parents to offspring through the genes in sperms and ova
• Different individual possesses different number and types of genes
Relationship between gene, chromosomes Relationship between gene, chromosomes
& DNA& DNA
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Genes in a
chromosomeMale Chromosomes
Exercise
1. What are chromosomes?
2. What are genes?
3. What is DNA stand for?
4. Name the two types of chromosomes.
5. Give the two types of sex chromosomes.
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Answers1. Chromosomes are a long, thread-like structure found in
the nucleus that store heredity information.
2. Genes are section of DNA that code for the production of protein and are a basic heredity unit.
3. DNA stand for deoxyribonucleic acid.
4. The two types of chromosomes are autosomes and sex chromosomes
5. The two types of sex chromosomes are XY for male chromosomes and XX for female chromosomes.
3.1.2 LIFE CYCLE AND CELL
DIVISION
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Life Cycle
• Life cycle started from a cell (a sperm from the
male & an egg from the female)
• Fusion of an egg and a sperm produce fertilize
egg.
• Fertilize egg undergoes cell division hundreds
of time to form embryo and undergoes cell
division hundreds of time again to form baby
• Baby grows by cell division into an adult
• Cell division continue to maintain the body
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Cell Division
• Has important roles in growth, repair &
reproduction in an organism
• Two types:
1. Mitosis – cells in different parts of human body,
root tips and shoot tips in plants (somatic cells)
2. Meiosis – cells in the sex organ to produce sex
cells/gametes
- in human & animals: ovary – female,
testes in male
- in plants: ovary – female, anther - male
1. Mitosis
• Cell division that produces genetically identical cells
• Produce 2 new cell, each containing an exact copy of
the DNA as in the parent cell
• The division continued, producing trillions of
genetically identical cells that make up your body
• The basis of asexual reproduction
• Replacement of cells and repairs of tissues
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Stages in Mitosis
• Stage 1
- Chromosomes
duplicated but remain
elongated
- Centrioles also
duplicate
• Stage 2
- Chromosomes begin
to condense and
become shorter,
thicken and visible
- Nuclear membrane
disappear
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• Stage 3
- Chromosomes arrange
themselves at the equatorial
plate
- Each chromosomes is
attached to a spindle fibre
through the centromere
• Stage 4
- Chromatids of each
chromosomes separates
become individual
chromosomes which
moves towards opposite
poles of the cell due to
contraction of the
spindle fibres
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• Stage 5
- Nuclear membrane reform
- Cytoplasm divided along
the equator
- Formation of 2 daughter
cells
Division of cytoplasm after completion of mitosis
in plant cell and animal cell
Plant cell Animal cell
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Division of cytoplasm after completion of
mitosis
in plant cell and animal cell
• Plant cell – cell wall will form between two
daughter nuclei to form two daughter cells
• Animal cell – cell membrane constricting
inward to form two daughter cells
Exercise
• Do activity 3.3 on page 29 in your practical
book
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Exercise
• Give a brief summary of life cycle.
• What are the importance of cell division?
• What are the types of cell division?
• What is mitosis?
• In which cell does the mitosis occur in plants?
Answers
• Life cycle started from a cell, the fertilize egg that
produce from the fusion of sperm and egg. This fertilize
egg will undergoes cell division to form embryo. The
embryo undergoes cell division again to form baby and
the baby will grow by cell division to become an adult.
• Have roles in growth, repair and reproduction process.
• Mitosis and meiosis.
• Mitosis is cell division that produce two genetically
identical cells.
• At the root tips and shoot tips of the plant.
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Things we learn today
• About life cycle
• Importance of cell division
• Types of cell division
• The meaning mitosis
• The stages in mitosis
2. Meiosis
• Cell division that produce 4 cells, each containing half
the number of chromosomes of the mother cell
• The cell produced are genetically different to the parent
cell and to each other
• Nuclei are haploid (n)
• Take place in sex organ to produce sex cell/gametes
• Can be divided into 2 main stages:
- First stage: the separation of homologous
chromosomes
- Second stage: separation of chromatids
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First Meiotic Division
• Stage 1
- Chromosomes replicated,
visible as long and coiled
threads
- Gradually become shorter
and thicker
- Centriole also duplicate
• Stage 2
- Pair of homologous
chromosomes lie side by
side
- Nuclear membrane
disappears
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• Stage 3
- The paired of homologous
chromosomes line up the
middle of the cell
- The homologous
chromosome attached to
spindle fibre
• Stage 4
- Two members of each pair of
homologous chromosomes
separate
- Moves towards opposite ends
of cell due to contraction of
spindle fibre
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• Stage 5
- Nuclear membrane
reform
- Cell membrane
constrict inward
- Formation of two
daughter cells
Second meiotic division
• Stage 6
- Each chromosomes appears as a pair of chromatids
- Nuclear membrane disappear
- Chromosomes line up at the middle of the cell
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• Stage 7
- Each pair of chromatids
separate from each other
- Moves towards the
opposite end of the cell
due to contraction of
spindle fibre
• Stage 8
- Nuclear membrane reform
- Cell membrane constrict inward
- Each chromatid become a
daughter chromosomes
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• Stage 9
- Formation of four daughter
cell each containing the
haploid (n) number of
chromosomes
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Importance of Meiosis
• Produces haploid cells that contain half number of chromosomes of body cell to maintain a constant number of chromosomes in offspring produced by sexual reproduction
• Produce genetic variation due to crossing overbetween homologous chromosomes, independent assortment and random fusion of two gametes during fertilisation.
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Exercise
• Do the Activity 3.3 on page 34 in the Science
Process Skills
Exercise
• What is meiosis?
• Where does meiosis occur in
a) plant?
b) animal?
• State the importance of meiosis.
• What is the importance of crossing-over
during meiosis?
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Comparison Between Mitosis
and Meiosis
Mitosis Meiosis•Maintain the chromosome number
•Halves the chromosome number
•Has one division cycle •Has two division cycle
•Produce two daughter nuclei •Produce four daughter nuclei
•Does not involve crossing over and independent assortment
•Involve crossing over and independent assortment
•Produce two daughter nucleithat are genetically identicalto one another and to the parent nucleus
•Produces four daughter nuclei that are genetically different from one another and from the parent nucleus
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The Similarities and
Differences Between
Mitosis and Meiosis
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Similarities
• Both are a type of cell division
DifferencesMitosis Meiosis
To produce new cells for growth and to replace damaged cells
Function To form gametes
In somatic cells Place where it occurs
In sex cells
Occurs Duplication process
Duplicate in Meiosis 1 but not in Meiosis 2
One Number of cell division
Two
Two Number of daughter cells
produced
Four
Identical to one another and to the parent nucleus
Genetic content
Different from one another and from the parent nucleus
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3.2 The Principal and Mechanism
of Inheritance
• Inherited characteristics - characteristics
that are passed from parents to offspring
through the genes in sperms and ova
• During fertilisation of an egg, 23
chromosomes comes from the father in
the sperm and 23 chromosomes come
from the mother in the ova
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3.2.1 Dominant Genes and Recessive Genes
• Dominant gene – gene that can show its trait
or characteristic controlled by it although it
exist with a recessive gene (strong gene)
• Recessive gene – gene that only shows its trait
or characteristic when a dominant gene is
absent (weak gene)
• Dominant gene control dominant
characteristic/trait whereas recessive gene
control recessive characteristic/trait
What are the types of
characteristics that we can
inherited from our parents?
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Characteristics Dominant
characteristics
Recessive
characteristic
Tongue rolling Ability to roll the
tongue
Inability to roll the
tongue
Ear-lobe Free earlobe Attached earlobe
Eye colour Brown eye Blue eye
Skin pigment Normal skin colour Absence of skin
pigment (albino)
Hair colour Black hair Brown hair
Hair type Curly hair Straight hair
Chin Projecting chin Receding chin
Nose Straight nose Upturned nose
Activity
• In pairs, list down your friends
characteristics.
• Determine whether your friend have
dominant characteristics or recessive
characteristics.
• Present your findings to the class.
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Conclusion
• More students have the dominant
characteristics than the students with
recessive characteristics.
Exercise
1. What is the meaning of inherited
characteristics?
2. What is dominant gene?
3. What is recessive gene?
4. Give two types of dominant characteristics.
5. Give two types of recessive characteristics.
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Answers1. Inherited characteristics are characteristics that are
passed from parents to offspring through the genes in sperms and ova
2. Dominant gene is gene that can show its trait or characteristic controlled by it although it exist with a recessive gene.
3. Recessive gene is gene that only shows its trait or characteristic when a dominant gene is absent.
4. Ability to roll the tongue, free ear-lobe, brown eyes, normal skin pigment etc.
5. Inability to roll the tongue, attached ear-lobe, blue eyes, absence of skin pigment (albino) etc
3.2.2 The Mechanism of Trait
Inheritance
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• Heredity - The study of inherited
characteristics
• Gregor Mendel – the first person who studied
heredity in a scientific way
• The body characteristics of organism are
inherited in different ways
• Monohybrid inheritance (simplest pattern of
inheritance) – inheritance controlled by a
single pair of genes.
• Character is determine by two contrasting traits (dominant and recessive)
• Example: Height – tall (dominant) and dwarf (recessive)
• Dominant trait represent by capital letter (T –tall) and recessive trait represent by small letter (t – dwarf)
• Gene is made of allele
• Homozygous – same allele (TT and tt)
• Heterozygous – different allele (Tt)
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• Genotype – genetic composition of particular
characteristic
• Phenotype – actual appearance of a
particular characteristic
Phenotype Genotype
Tall (pure strain) TT (homozygous dominant)
Tall (monohybrid)
Tt (heterozygous)
Dwarf (pure strain)
tt (homozygous)
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Activity
• Discuss Activity 3.3 on page 62 and 63 in your
text book.
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3.3. Sex Determination and Occurrence
of Twins in Human Beings
Sex Determination in Human
Being
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3.3.1 Sex Chromosomes
• Determine the sex of an individual
• Two types:
1. XX – in female
2. XY - male
• In female an ovum contain 22 autosomes and an X chromosome
• In male there are two types of sperm produced, each contain either 22 autosomes and an X chromosome or 22 autosomes and a Y chromosome
Chromosomes In Human
Male Female
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• The sex of a baby is determine by the type of
sperm that fuses with the ovum during
fertilisation:
a) When an ovum is fertilised by a sperm
containing an x chromosomes, the resulting
zygote has a genotype of XX and will develop
into a girl
b) When an ovum is fertilize by a sperm with a Y
chromosomes, the zygote has a genotype of
XY and will develop into a boy
Parents: Male 44+XY Female 44+XX
Gametes: 22+X 22+Y 22+X 22+X
Offspring
genotype: 44+XX 44+XY 44+XY44+XX
Offspring
phenotype: Girl BoyGirl Boy
Genotype ratio – 2XX : 2XY
Phenotype ratio – 2 Girl : 2 Boy
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Occurrence of Twins in Human
What are twins?
• A type of multiple birth.
• Two foetus develop in the womb at the
same time
• Two types of twins:
1. Identical twins
2. Non-identical twins
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1. Identical Twins
• Formed when one egg is fertilised by one sperm
• After fertilisation, the zygote splits into two
• Two embryos are formed from one zygote, and each develops as a separate baby
• Both children inherit the same genes
• Alike in appearance
• Same sex
• Share a common placenta
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2. Non-identical Twins
• Also known as fraternal twins
• Form when two separate eggs are fertilised by two sperms
• Two embryos are formed from two separate zygote, and each develop as a separate baby
• Both children inherit different genes
• Not alike
• Sex maybe the same or may be different
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Siamese Twins
• A type of identical twins
• Form when two embryos do not separate
completely during the development stage
• Attached to each other at the stomach, head,
or even share the same internal organs (liver,
heart or brain)
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Which picture show the identical twin and
non-identical twins?
A B
Identical twins Non-identical twins
Activity
• In your group, discuss to complete the graphic
organizer shown on page 66 in your text book.
• Copy the complete graphic organizer into your
note book.
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Answers
Similarities
A type of multiple birth
Two fetus develop in the womb at the
same time
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Differences
Identical Twins Non-identical
Twins
One Number of ova involved Two
One Number of sperms
involved
Two
One Number of zygotes
produced
Two
One Number of placenta
formed
Two
Same Sex of twins Same or
different
Same Genotype of twins Different
Exercise
1. What are twins?
2. Explain in brief how identical twins occur in
human.
3. Give a similarity between identical twins and
non-identical twins.
4. Give two differences between identical twins
and non-identical twins.
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3.4 MUTATION
What is mutation?
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• A sudden change of chromosomes or genes that can change human traits
• Change in the amount or the chemical structure of DNA
• Occur in somatic or gamete cells
• Mutation in somatic cells cannot be passed but mutation in gamete cells are inheritable
• A permanent process
• Occur spontaneously under natural condition or induced by substances called mutagens
• Organism that have undergone mutation are called mutants.
Types of Mutation
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• Two types:
1. Chromosome mutation
� occurs when changes take place to the
structure of the chromosomes or to the
number of chromosomes
� changes in the structure may caused by
deletion (lost of genes), inversion,
translocation and duplication
� changes in the number of chromosome may
caused by additional or deletion of one or more
extra sets of chromosomes
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2. Gene mutation
� change to the chemical structure of a gene
� causes some inherited diseases such as
albinism, sickle-cell anaemia, haemophilia
and colour blindness
Examples of Mutation
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1. Male with genotype 44+XXY (Klinefelter’ssyndrome) is sterile, mentally handicapped and has female characteristics due to the extra X chromosome
2. Male with genotype 44+XYY usually has high level of testosterone, have severe acne and tall in height due to the extra Y chromosomes
3. Female with genotype 44+XO (Turner’s syndrome) is sterile because her reproductive organs are not functional due to the fact that she has only one X chromosome
4. Female with 44+XXX tendency to be tall and a higher incidence of below- normal intelligence due to the extra X chromosome
5. Down’s Syndrome (Mongolism)
• extra chromosomes to the 21st
• have a short and stocky body, slanting eye,
broad face and stubby nose
• mentally retarded and have heart defects
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6. Haemophilia – the blood fails to clot therefore
suffers prolonged bleeding from cuts caused by
gene mutation at the X chromosome
7. Colour blindness – cannot identify colours due
to the mutation at the X chromosome which
cause defects in one or more of the three types
of cones in the retina
8. Sickle-cell anaemia – mutation that changes
the shape of the red blood cell to become
sickle-shaped which have low efficiency in
transporting oxygen and leading to anaemia
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Albinism in human Albinism in animal
9.9.Albinism Albinism –– mutation that cause the gene mutation that cause the gene
which controls the skin pigment fails to which controls the skin pigment fails to
produce pigment melanin therefore will cause produce pigment melanin therefore will cause
a person have pale skin, light hair and pink a person have pale skin, light hair and pink
eyeseyes
Exercise
• Do the exercise in the Process Skill book page
42.
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Things that we’ve learned today
• The meaning of mutation.
• The types of mutation.
• Examples of mutation.
Causes of mutation
• Spontaneously or induced by mutagens
• Types of mutagens
1. Industrial chemical – benzene, formaldehyde, mustard gas, nitrous acid, diphenylamine and bromouracil
2. Radiation – ultraviolet light, X-rays and nuclear waste (alpha, beta and gamma)
• The higher the dosage of radiation, the higher rate of mutation
3. Increase of temperature outside the normal temperature range
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Advantages and Disadvantages of
Mutation
Advantages Disadvantages
1. 1.
2. 2.
3.
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Advantages Disadvantages
1. Lead to variation 1. Physical deformities
- polydactyl traits →
extra fingers or toes
2. Creation of new species 2. Genetic disease
- gene mutation→
colour blindness and
albinism
- chromosome mutation
→ Down’s Syndrome
and Turner’s
Syndrome
3. Precursor to the
evolution process
Exercise
1. What is mutation?
2. Give types of mutation.
3. Give two examples of mutation.
4. What cause mutation?
5. Give one advantage and disadvantage of
mutation.